When an air-conditioner is heating, the evaporator will defrost, and too thick frost will reduce the heating efficiency. Therefore, the evaporator is required to be defrosted. The existing defrosting mode is using a four-way valve to change air flow direction to defrost the evaporator wholly. When in defrost, the condenser becomes an evaporator, and heating is forced to stop thus reducing the heating efficiency of the air-conditioner.